CI: Update to Ubuntu 18.04 to 20.04

Github actions will drop support for 18.04 April 1, 2023. There
are now sporadic test cancelations due to warning brownouts, and
the NumPy 1.24.x release will overlap that date, so increase the Ubuntu
version.

The main change here is that there is no support for gcc-6 in Ubuntu
20.04, so testing with that version is also dropped.

.github/workflows/build_test.yml

@@ -81,7 +81,7 @@ jobs:
   old_gcc:
     needs: [smoke_test]
     # provides GCC 6, 7, 8
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:
@@ -99,14 +99,7 @@ jobs:
         pythonx -m pip install --upgrade pip setuptools wheel
         pythonx -m pip install -r test_requirements.txt
     - name: Install Compilers
-      run: sudo apt install g++-6 g++-7 g++-8 -y
-    - name: Build gcc-6
-      run: |
-        export CC=/usr/bin/gcc-6
-        export CXX=/usr/bin/g++-6
-        pythonx setup.py install --user
-    - name: Runtests gcc-6
-      run: pythonx runtests.py -n
+      run: sudo apt install g++-7 g++-8 -y
     - name: Build gcc-7
       run: |
         export CC=/usr/bin/gcc-7
@@ -199,7 +192,7 @@ jobs:
 
   full:
     needs: [smoke_test]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       USE_WHEEL: 1
       RUN_FULL_TESTS: 1